The present invention relates to aeration diffusers used in wastewater treatment tanks, and more specifically to the piercing pattern on the elastomeric membranes of the diffuser assemblies.
Aeration systems are used in wastewater treatment tanks to introduce oxygen into the wastewater. The wastewater is typically contained in treatment tanks, and air is forced through diffuser assemblies to produce fine bubbles that flow through the wastewater. Diffusing a high volume of air or oxygen into the wastewater in the form of fine bubbles facilitates biological growth during the waste treatment process. Supplying air into the treatment tank also serves to prevent sedimentation of the wastewater within the treatment tank. The treatment tank usually includes a network of air distribution piping for transferring air to the diffuser assemblies. The network of air distribution piping typically includes a drop pipe extending from an air supply to a manifold that is submerged within the wastewater. The submerged manifold is connected to a plurality of submerged distribution pipes that are also submerged within the wastewater and generally arranged in a parallel configuration along the bottom of the treatment tank when such tank is of rectangular design. Each distribution pipe typically supports a number of diffuser assemblies such that the diffuser assemblies are also submerged within the wastewater along the bottom of the tank.
Each diffuser assembly includes a diffuser body and includes a membrane coupled to the diffuser body. The membrane includes a perforation pattern that generates the fine bubbles. Diffuser assemblies can include circular or rectangular diffuser bodies and include corresponding circular or rectangular membranes. Many different types of membranes are known and are described in detail below.
The perforation pattern of circular membranes can be either segmented or unsegmented. Segmented patterns divide the surface area of the circular membrane into a number of equal-sized, pie-shaped segments. Each of the segments includes the same or substantially the same piercing pattern. Segmentation allows consistent spacing between and across rows regardless of the radial distance from the center of the membrane. Some known segmented patterns include square or rectangular piercing patterns while others include parallelogram piercing patterns. As shown in FIG. 1, the square or rectangular piercing pattern is generated by creating slits in a square or rectangular pattern across the segment. The parallelogram piercing pattern is generated by creating punctures in a parallelogram pattern across the segment. Slitting is different from puncturing in that a slit is an extended cut into the membrane and a puncture is a generally circular hole through the membrane. Some square or rectangular patterns use curved slits and orient the curved slits in opposite facing directions in adjacent rows.
With further reference to FIG. 1a, the line b extending between midpoints of adjacent slits in a common row is the same length as the line a extending between midpoints of adjacent slits of adjacent rows. Specifically, the lines a, b extend between common points (e.g., center points) of the slits. Also, the angle A defined between the lines a, b equals 90 degrees. The common length and 90 degree angle define the square pattern. A rectangular pattern is defined when the 90 degree angle exists, but the lines a, b are not equal. In one embodiment, the perpendicular distance d between adjacent rows is 0.100 inches and the lines a, b are also 0.100 inches. This allows for a maximum bubble diameter D of up to 0.100 inches, at which diameter adjacent bubbles touch and likely coalesce into a single larger bubble which transfers oxygen less effectively than two smaller uncoalesced bubbles.
On round membrane diffusers, unsegmented patterns do not divide the piercing pattern into repeatable angular segments. Known types of unsegmented patterns include a circumferential slitting pattern, a spiral slitting pattern, and a random puncturing pattern. The circumferential slitting pattern includes rows of slits spaced at radial intervals across the surface area of the circular membrane. The relative spacing between adjacent slits is not consistent and varies depending upon the location of the slit on the membrane. The spiral slitting pattern is similar except that it includes a single row of slits that gradually increases its radial position around the circular membrane such that the row wraps or spirals around itself. The random puncturing pattern does not include any repeatable pattern and therefore there is no consistent spacing in any direction between adjacent punctures. To accommodate a maximum bubble size of 0.100 inches before coalescence, such random patterns must accommodate the closest adjacent slits, and are forced to use a row-to-row spacing of 0.100 inches. This pattern is inefficient in the sense that over most of the membrane the center-to-center distance between adjacent slits is greater than 0.100 inches.
Rectangular membranes are generally unsegmented and include a piercing pattern that is continuous across the surface area of the membrane. Some examples of piercing patterns for rectangular membranes include diamond, parallelogram, square or rectangular slitting patterns. With reference to FIG. 1A, a diamond pattern is defined when the base angle A is 60 degrees and the lines a, b are the same distance. In contrast, a parallelogram has a base angle greater than or less than 60 degrees and lines a, b are not equal distances.